Communications radios, such as those employed in mobile cellular handsets and similar devices, must be capable of receiving very small signals with small relative bandwidth, and are thus sensitive to periodic disturbances of any kind whose frequency lies within the wanted received channel. In addition to external sources of interference, most radios contain internal frequency generators. These internal generators include local oscillators used both for creating signals to be transmitted, and converting received signals to a more convenient lower frequency (the intermediate frequency or IF). Most modern radios contain digital circuitry, whose operation is synchronized by one or more clock oscillators. In addition, in many cases, circuitry within the radio requires DC supply voltages differing from the main power supply or supplies (e.g. a battery), requiring conversion and regulation. DC-DC conversion is often performed by switch-mode converters, which provide superior efficiency and other benefits relative to linear regulators. Switch-mode converters, however, may generate periodic signals, both through their internal operations and through imposition of corresponding periodic variation (ripple) in the voltage they deliver to a load, which can also represent a source of internal interference.
Undesired periodic signals are generally referred to as spurious signals, or spurs. Internal oscillators may generate spurs not merely at their operating frequency, but at other frequencies, through two basic mechanisms. The first mechanism is the creation of harmonics of the fundamental frequency. Harmonics may be present in the source signal if it is not sinusoidal. For example, a square wave may be used as a reference signal for timing control in a switch-mode converter. Square waves are composed of a signal at the fundamental frequency, plus signals at odd harmonics thereof, decreasing in inverse proportion to the harmonic multiplier. Internally, a switch-mode converter often employs a sawtooth ramp to control its switch state, producing a similar spectrum with both even and odd harmonics. Finally, the output waveform of, for example, a buck converter is a triangle wave, with alternating increasing and decreasing currents whose slope is controlled by the output inductor and capacitor; such a wave contains odd harmonics whose amplitude decreases with the square of the harmonic multiplier.
In addition to harmonics present in the source signal, any nonlinear circuit response, such as a limiting process or a mixing process, can produce additional harmonics of an input signal. Finally, nonlinear processes can mix two signals at differing frequencies together, in principle resulting in signals with frequency components at all frequencies of the form:fspur=n·f1+m·f2 where fspur is the spurious signal frequency, n and m are integers (which may be negative), and f1 and f2 are the frequencies of the two signals participating in the nonlinear mixing process. While the discussion of the exemplary method focuses on integer harmonics, non-integer harmonics are also possible in special cases. For example, DC-DC switch-mode converters can display sub-harmonic oscillations near their upper or lower duty cycle limits, or when interferers enter the control circuitry. Precautions may be taken in design and operation to avoid sub-harmonic oscillations, or their presence can be taken into account in the method described below.
Spurious signals have two general classes of deleterious effects. The first and usually most important is desensitization of the receiver. A spurious signal at the frequency the receiver is tuned to may impair the ability to recognize and decode the wanted signal. Extremely small spurious signals can cause problems. For example, consider a WCDMA receiver with sensitivity is limited by thermal noise. The signal bandwidth is about 3.8 MHz, so thermal noise entering the receiver at room temperature is about −174 dBm/Hz+66 dB≈−108 dBm. If the receiver noise figure is 8 dB, the noise floor in the receiver channel is about −100 dBm (that is, 10−13 Watts). Any spurious signal larger than −100 dBm will degrade receiver sensitivity. In a superheterodyne receiver, in which the received signal is converted to a lower intermediate frequency (IF), spurious signals lying on the converted frequency may also interfere with the wanted signal. IF signal levels are higher than RF signals, since some gain has usually been applied to the signal before conversion, but IF frequencies are lower than RF frequencies and thus more susceptible to interference from harmonics of local signals.
The second class of problems arises from spurs which affect the transmitted signal, for that class of radios which act as transceivers. Spurious transmitted signals that are outside the intended transmission channel may interfere with the ability of neighboring radios to receive their wanted signals, and are thus often subject to regulatory requirements. Spurious transmitted signals within the intended channel may degrade the quality of the transmitted signal, as measured by the error vector magnitude (the difference between the intended and actual phase and amplitude of the transmitted signal), carrier feedthrough, or other measures of signal fidelity. Degraded signal quality gives rise to high link error rates and degraded performance.
Traditionally, DC-DC converters employed in radios have operated at switching frequencies from a few tens of kHz to a few MHz. Resulting spurious harmonics extend up to a few tens to a few hundred MHz., and are of special concern within the IF chain, where the wanted channel may lie on a harmonic frequency. When mixed with a carrier, as may occur due to residual nonlinearity when a switching regulator is used to supply DC power to a transmit power amplifier, a number of spurs near the carrier frequency can be produced. Various approaches are known in the art to minimize spurs due to switch-mode converters. Converter output ripple can be reduced by using multiple conversion blocks operating with different phase relationships arranged to partially or completely cancel output ripple. Output ripple may be reduced using a supplementary linear regulator in shunt with the switch mode regulator. The impact of spurs resulting from a switched-mode regulator may be reduced by varying the switching frequency in various ways to spread the spurious output over a wider channel than that of interest. Finally, the frequency of local clocks such as that of a switched-mode converter may be intentionally and dynamically changed to avoid wanted channels.
Increases in switching frequencies provide a number of potential advantages for switch-mode converters, including reduced size and cost of external components and faster transient response. Recent developments have led to switching frequencies of tens to hundreds of MHz, as described in the copending application Ser. No. 12/646,213, titled “Stacked NMOS DC-to-DC Power Conversion”, filed Dec. 23, 2009, which is herein incorporated by reference. However, when switching frequency is increased, careful attention must be paid to resulting spurious signals. Harmonic frequencies increase proportionally to the increase in the fundamental, and may extend up to several GHz, so that they are more likely to lie directly within the wanted receive channel rather than merely within the IF channel. In the case of frequency-division duplexed (FDD) radios with simultaneous transmission and reception, the standard operating procedure for CDMA and WCDMA cellular handsets, mixing products with the carrier frequency, of particular importance in the case where the switched-mode converter supplies an RF power amplifier, may lie on the paired receive channel. When this occurs, desensitization will result regardless of the current radio, channel in use, since the mixing product with the carrier, and the paired receive channel, will both maintain a constant offset from the carrier as it changes frequency. The importance of switching frequency selection for high-frequency converters has been recognized, but only in the context of avoiding harmonics within the transmit band. A more thorough and systematic approach is required to minimize the deleterious effects of spurious signals, so that the benefits of high switching frequency may be realized.
It is desirable to have methods and apparatuses for operating switching of a voltage regulator at a switching frequency that provides a desired level of spurious signal degradation of receive and/or transmit signals of a transceiver that uses the voltage regulator.